In a radio link of an uplink radio access unit according to LTE (Long Term Evolution) whose specification is formulated according to 3GPP (3rd generation partnership project) and whose functions are kept being extended, transmission timing control is applied to uplink radio signals that are transmitted from radio terminal devices. In a radio base station, reception timing of uplink signals that are transmitted from the radio terminal devices is measured and the radio base station transmits a control signal containing a transmission timing adjustment amount to each of the radio terminals such that a difference between the measured reception timing and reference timing is equal to or smaller than a given specified value (for example, the length of a cyclic prefix or a guard interval that is used for uplink signals).
Performing such transmission timing control achieves low mutual interference between signals from respective radio terminals that are transmitted via uplink radio links, enables improvement in transmission characteristics and thus improves data throughput characteristics in radio uplinks. Physically, it is assumed that radio synchronization is achieved when the aforementioned difference between the reception timing and the reference timing is equal to or smaller than the specified value. In order to maintain radio synchronization reliably, it is ideal that the radio base station constantly performs reception timing measurement on signals from the ratio terminals and transmission of control signals for transmission timing adjustment based on the result of the measurement to the radio terminals. With an increase in consumption of radio resources, however, power consumption of the radio terminals also increases.
In LTE, timers are used in order to maintain radio synchronization more easily. A radio terminal initializes and resets a timer on receiving a control signal for transmission timing adjustment from the radio base station, starts elapsed time measurement from 0 and, until the elapsed time reaches a given time of which the radio terminal is notified by the radio base station separately, regards that uplink radio synchronization is maintained. When the elapsed time exceeds the given time, the radio terminal regards that radio synchronization is not maintained (asynchronous). When a new control signal for adjusting transmission timing is received before the elapsed time reaches the given time, the timer is reset and the elapsed time is measured from 0.
Patent Literature 1: Japanese Laid-open Patent Publication No. 08-130765
Patent Literature 2: Japanese National Publication of International Patent Application No. 2015-534341
When the elapsed time in the timer in a radio terminal exceeds the given time and the radio terminal is regarded as entering an asynchronous state, periodic radio resources that have been assigned to the radio terminal (such as a control channel used to report radio characteristics of a radio downlink, a control channel used to request assignment of radio resources of a radio uplink, and a pilot signal that is transmitted to evaluate radio characteristics of the radio uplink) are all released. Furthermore, control signals for granting transmission of uplink signals are not transmitted to the radio terminal. In order for a radio terminal in which the elapsed time of the timer exceeds a given time and thus the radio terminal enters the asynchronous state to transmit an uplink signal, a radio synchronization establishment process starting with transmission of random access signals is to be performed to re-establish radio uplink synchronization. As a result, the data throughput decreases.
In order to avoid such a situation, it is necessary that a control signal for adjusting transmission timing is transmitted to each radio terminal before the elapsed time in the timer of the radio terminal reaches the given time. When radio resources are consumed to transmit control signals for adjusting transmission timing and the number of radio terminals increases, the amount of consumption of radio resources can become considerable. As a control signal for adjusting transmission timing does not contain user data, frequent transmission of control signals for adjusting transmission timing leads to deterioration of throughput characteristics with respect to user data.
In a different viewpoint, when the synchronous state of a radio uplink is determined based on the elapsed time of timers, matching with respect to elapsed time is needed between the timer in each radio terminal and a timer corresponding to each radio terminal on the side of the radio base station, which leads to an increase in difficulty in system control and in circuit scale.